Cigarette smoking in pregnancy and fetal hyperviscosity.

Abstract

The effects of maternal cigarette smoking on fetal blood viscosity and its major determinants were investigated in an effort to determine if there might be a relation between the intrauterine growth retardation associated with cigarette smoking and fetal hyperviscosity. Blood was collected from the umbilical vein at birth in 40 infants born to mothers who had smoked more than 20 cigarettes a day throughout pregnancy and from 40 matched nonsmoking controls. Blood and plasma viscosity packed cell volume plasma fibrinogen concentration and erythrocyte deformability were measured by standard techniques and statistical comparison of the differences in means between the 2 groups made using Students t test. Cigarette smoking was associated with a 30% increase in fetal blood viscosity this rise being compounded of a 12% increase in packed cell volume and an 18% fall in erythrocyte deformability. Neither plasma viscosity nor plasma fibrinogen values were altered. The mean birth weight was 318 gm lower in infants born to smoking mothers. When adult smokers become pregnant the large physiological variation in blood viscosity factors obscure the effects of smoking in maternal blood. This is not the case in the fetus. In the extensive capillary microcirculation of the fetal placental villi raised blood viscosity and reduced erythrocyte deformability would be apt to reduce blood flow significantly in accordance with Poiseuilles law. Reduced intravillus perfusion would combine with reduced intervillus blood flow on the maternal side of the placenta owing to the effects of nicotine and with the already reduced oxygen availability in both maternal and fetal blood to cause fetal hypoxia stimulating fetal erythropoiesis with a further increase in blood viscosity leading to a vicious circle of decreasing flow and further hypoxia. The study indicates a new pathogenic pathway for the deleterious effects of maternal cigarette smoking on fetal growth and development in pregnancy.